A dot-and-cross diagram is a bookkeeping tool: it shows where each outer electron came from and where it ended up. Get the counting right and ionic compounds, multiple bonds and dative bonds all follow the same rules.
Dot-and-cross diagrams show outer-shell electrons only, using dots for one atom's electrons and crosses for the other's. For an ionic compound, draw each ion separately in square brackets with its charge, showing the electrons transferred. For a covalent molecule, place each shared pair in the overlap between atoms (one dot and one cross), two pairs for a double bond and three for a triple; draw lone pairs on the atom they belong to. In a dative bond both electrons of the shared pair carry the donor's symbol. Check the finish: every atom (except H, B and expanded-octet atoms) has eight outer electrons, and the total matches the number you started with, adjusted for any charge.
Use the dot-and-cross simulator to build H2O, CO2, N2 and NH4+ step by step. Before each step, predict where the next pair will go.
For ionic compounds show the electron transfer, not sharing. The diagrams are drawn as separate ions.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
In a dot-and-cross diagram of methane, CH4, how many lone pairs does carbon have?
How many lone pairs are there on the oxygen atom in a water molecule?
In the dot-and-cross diagram of sodium chloride, the chloride ion is drawn with:
How many electrons are shared between the two nitrogen atoms in N2?
These are the diagrams that come up again and again in exam papers. Each one tests a different rule.
| Species | What to show |
|---|---|
| N2 | three shared pairs between the N atoms, one lone pair on each |
| CO2 | two shared pairs on each side of C; two lone pairs on each O |
| NH4+ | four shared pairs; in the dative N–H bond both electrons carry nitrogen's symbol; square brackets and + charge |
| CO | three shared pairs, one of them with both electrons from O; one lone pair on each atom |
| Al2Cl6 | two bridging Cl atoms, each forming one normal bond and one dative bond to Al |
| BF3 | only six electrons around B, three lone pairs on each F |
Count the total before and after. NH4+ has 5 + 4 − 1 = 8 outer electrons, which is four pairs and no lone pairs. The most common mistakes are leaving the lone pairs off the outer atoms, forgetting the square brackets and charge on an ion, and drawing a dot and a cross in a dative bond where both electrons should come from the donor.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
How many electrons in total are in the outer shells of all the atoms in the ammonium ion, NH4+?
In the dot-and-cross diagram of carbon monoxide, how many of the six shared electrons come from the oxygen atom?
Which feature is correct in the dot-and-cross diagram of CO2?
How many lone pairs are there in total in one molecule of BF3?
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